Tank columns moving on Syria's commercial capital along with air strikes represent serious escalation of Assad's efforts to crush rebellion, State Department says.

AMMAN/BEIRUT - Syrian President Bashar Assad's forces renewed a ground and aerial bombardment of Aleppo on Friday, extending efforts to crush rebels in Syria's commercial capital in what the United States said it feared could become a massacre.

Insurgents targeted army roadblocks and security installations, with both sides avoiding close-quarters warfare in the city of 2.5 million people, Syria's biggest urban center.

The US State Department said credible reports of tank columns moving on Aleppo, along with air strikes by helicopters and fixed-wing aircraft, represented a serious escalation of Assad's efforts to crush a rebellion that began 16 months ago.

"This is the concern: that we will see a massacre in Aleppo, and that's what the regime appears to be lining up for," State Department spokeswoman Victoria Nuland said.

Troops stationed on the outskirts of Aleppo unleashed barrages of heavy-caliber mortar rounds on the western neighborhoods of Saladin, al-Sukkari and al-Fardos, while Russian MI-25 helicopter gunships struck al-Sakhour in the east with rockets, several opposition activists in the city said.

In the first reported casualty on Friday, a man of about 60 wearing a traditional white prayer outfit was killed near a park in Saladin. His body was placed in a mosque pending identification.

Thirty-four people were killed in Aleppo and its environs on Thursday, according to opposition activists keeping a tally of casualties in the northern city.

"The rebels have so far been nimble, and civilians have mostly been the victims of the bombardment," said activist Abu Mohammad al-Halabi, speaking by phone from the city.

"There is lots of internal displacement, and schools have been turned to makeshift shelters that are packed. One shell hitting a school will result in a catastrophe," he said.

"The regime is massing troops and tanks at the entrances of Aleppo, but it seems it is for now content with bombarding the city, with the rebels constantly on the move."

Majed al-Nour, another activist, said rebels attacked a security outpost in the neighborhood of Bustan al-Joz, which is close to the Aleppo city center, on Thursday.

"The rebels are present in the east and west of the city, and have a foothold in areas of the center. The regime forces control the entrances of Aleppo and the main thoroughfares and commercial streets and are bombarding the residential districts that fell into rebel hands," he said.

Nour said tens of thousands of people had fled Aleppo to nearby northern rural regions close to Turkey from which the Syrian army has withdrawn in recent weeks to focus on urban areas where relatively lightly armed rebels have hunkered down.